Due to its great theoretical capacity (147 mAh g−1) and high operating potential (4.7 V vs Li+/Li), Co-free spinel LiNi0.5Mn1.5O4 (LNMO) is one of the most promising thin film cathodes allowing designing Li-ion micro-batteries with a high specific energy. In this work, the Li extraction–insertion mechanism in sputtered LNMO thin films is investigated by X-ray diffraction and Raman spectroscopy during the first electrochemical cycle. A one-step phase transition involving two cubic phases is revealed, consisting of a wide solid solution region (0.3 ≤ x ≤ 1 in LixNMO) and a narrow biphasic domain (0 < x ≤ 0.3). Remarkably, significant variations are observed in the Raman spectra, which are linked to the activity of the Ni redox system at 4.7 V. It is demonstrated that an appropriate analysis of the bands corresponding to pure Ni–O stretching modes leads to an accurate estimation of the electrode states of charge and depth of discharge, which opens the way for a reliable quantification of the self-discharge phenomenon. The mechanism of Li extraction insertion here pictured for the first time for LNMO thin layers is consistent with their disordered nature and accounts for their good electrochemical performance.